The invention relates to a process for the preparation of a hydrocarbon mixture rich in aromatics from a hydrocarbon mixture that is low in aromatics and which boils in the gasoline range.
Hydrocarbon mixtures rich in aromatics are used on a large scale as gasoline. Separate aromatic compounds may be isolated from them by distillation and extraction, such as benzene, toluene, the xylenes, and ethylbenzene which are used on a large scale as raw materials for the chemical industry. Until recently, hydrocarbon mixtures rich in aromatics which were suitable for use in the above-mentioned applications were mainly prepared by catalytic reforming of hydrocarbon mixtures low in aromatics and boiling in the gasoline range. The latter hydrocarbon mixtures consist substantially of unbranched and branched paraffins and naphthenes. In the catalytic reforming of these hydrocarbon mixtures, a number of reactions occur of which the principal ones are isomerization and cyclization of paraffins, conversion of five-membered ring naphthenes into six-membered ring naphthenes and dehydrogenation of six-membered ring naphthenes into aromatics. Although one of the principal reactions in catalytic reforming is the conversion of unbranched paraffins, the reformate still contains a considerable quantity of these compounds. When the reformate is used as motor gasoline, the presence of these compounds is undesired because of their low octane number. In the past, it has already been proposed to remove unbranched paraffins present in reformates by selective cracking. A drawback is here, however, that cracked products which arise from the unbranched paraffins, boil below the gasoline range so that this treatment is detrimental to the gasoline yield. The use of the reformate as the feed for the preparation of C.sub.8 aromatics by distillation and extraction involves the serious drawback that the remaining hydrocarbon mixture is less suitable as gasoline because it does not contain any C.sub.8 aromatics which are very valuable gasoline components. It would therefore be desirable to have available a process which offers the possibility of converting certain components present in the reformate into C.sub.8 aromatics so that the C.sub.8 aromatics originally present in the reformate are preserved as gasoline components.
In an investigation concerning the preparation of hydrocarbon mixtures rich in aromatics which are suitable as motor gasolines with a high octane number or as the source for the recovery of C.sub.8 aromatics, starting from hydrocarbon mixtures low in aromatics with the use of catalytic reforming, the applicants have found that the above problems can be solved by subjecting the reformate to a conversion using as the catalyst certain crystalline silicates which have recently been synthesized as described in Netherlands patent application No. 7,613,957 incorporated herein by reference.
It has been found that by contacting a reformate or a fraction thereof at elevated temperature with a catalyst containing one of these crystalline silicates, unbranched paraffins present therein and monoethyl paraffins are cracked, and that the fragments so obtained unite by alkylation with the aromatics present in the reformate. In this way, a considerable increase in octane number is obtained without significantly cutting the gasoline yeild. An attractive aspect of the process is that, in addition to unbranched paraffins, monoethyl paraffins also disappear from the reformate since the latter paraffins have an octane number higher than the unbranched paraffins have, but still considerably lower than the aromatics and highly branched paraffins have. It has further been found that by contacting a heavy fraction of a reformate, which fraction contains C.sub.9 +aromatics at elevated temperature with a catalyst containing one of these crystalline silicates, disproportionation of the C.sub.9 +aromatics occurs, whereby C.sub.8 aromatics are formed in high yield. The C.sub.8 -fraction of the reformate which comprises the C.sub.8 aromatics formed in the catalytic reforming makes an excellent motor gasoline.